Keying system



Signal Amplitude Jan. 22, 1952 JACOB 2,583,146

KEYING SYSTEM Filed Aug. 6, 1949 Grid Voltage Fig.6.

Time WITNESSES: Fl 9.

547% Mork I.Jucob- 22a- 4 I A TORNEY INVENTOR Patented Jan. 22, 1952 KEYING SYSTEM MarkI. Jacob, Baltimore, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Y Application August 6, 1949, Serial No. 108,961

This invention relates to the electronic keying of resistance coupled speech amplifiers used in radio transmitters, and it has as an object the maintenance of a steep wave front at the output of such an amplifier during electronic keying thereof, without the presence of spikes or clicks.

Resistance coupled amplifier circuits are commonly used in the speech amplifiers of radio 4 Claims. (Cl. 250-27) transmitters. In the past it has been impractical to electronically key such amplifiers for the reason that the time delays caused by the time constants of the resistance coupled circuits limit the keying speed to an impractical number of words per minute and have caused spikes at the grids of the amplifier tubes following the keyed tubes, resulting in objectionable audible clicks. For this reason it has been the practice in the past where resistance coupled amplifiers were used, to either key such amplifiers with relays, or to key audio frequency oscillators ahead of the amplifiers. Keying by relays involves additional first cost and increased maintenance expense, and the keying of audio oscillators has resulted in undesired lilts in the outputs of the amplifiers.

According to this invention, instead of keying a resistance coupled amplifier by applying blocking bias pulses to the control grid of a single vacuum tube which is resistance coupled to a following tube, out-of-phase blocking bias pulses are applied simultaneously to the control grids of two similar tubes, the plates of which are interconnected and the cathodes of which are interconnected. Thus one tube conducts while the other is cut-off. This results in a constant D. C. voltage at the plates of the tubes during the keying, and'this, in turn, results in constant bias voltage and therefore a square wave of audio frequency potential at the plate of the following tube.

The invention will now be described with reference to the drawing, of which:

Figure l is a circuit schematic illustrating how resistance coupled amplifiers have previously been electronically keyed;

Fig. 2 is a graph illustrating the D. C. voltages at the grid of the keyed tube of Fig. 1 during the keying;

Fig. 3 is a graph illustrating the D. C. voltages at the plate of the keyed tube of Fig. 1 during the keying;

Fig. 4 is a graph illustrating the D. C. voltages at the control grid of the tube following the keyed tube of Fig. 1 during the keying;

Fig. 5 is a. chart illustrating the wave form 2 during keying of the audio frequency signal at the plate of the following tube of Fig. 1;

Fig. 6 is a circuit schematic of one embodiment of this invention, and

Fig. '7 is a chart illustrating the wave form during keying of the audio frequency signal at the plate of the tube following the keyed tubes of Fig. 6.

Referring now to Fig. 1 of the drawing, the triode, input tube [0 is resistance coupledto the triode, following tube II. The audio frequency signal input is applied between the grid of the tube In and its grounded cathode. The keying pulse is applied through the grid resistor l2 to the grid of thetube In and through ground to its cathode, and has the shape shown at A. Normally the tube l0 is biased to cut-off so that no signal will appear at its plate, and no plate current will fiow through the plate resistor 13.

The D. C. biasingvoltages at the grid of the tube It] resulting from the keying pulses are shown by Fig. 2. The positive pulse of voltage at the grid of the tube l0 when the key of the keyer is down, causes the tube [0 to conduct so that the signal appears at its plate during the period T. The flow of plate current through the plate resistor l3 causes a voltage drop therein resulting in the D. C. voltage at the plate of the tube I0 decreasing as shown by Fig. 3. This causes a D. C. pulse to be applied to the grid of the tube II. Due, however, to the time constant of the coupling capacitor l4 and the grid resistor l5, the D. C. voltage appearing at the grid of the tube II is that shown by Fig. 4. This voltage represents a bias change on the grid of the tube I l which results in its output signal having the wave shape shown by Fig. 5. This varying output not only limits keyingspeed noticeably, but also objectionable clicks are produced by the spikes shown by Fig. 4 at the end of the pulses. These undesired eifects are eliminated by the circuit shown by Fig. 6 which will now be described.

The circuit of Fig. 6 is similar to that of Fig. 1 except that the tube l6 has been added, with its cathode and plate connected to the cathode and plate, respectively, of the tube It], and the keying voltage source H which supplies the pulse A to the grid of the tube It, supplies a similar out-of-phase pulse B, through the grid resistor I8, to the grid of the tube Hi. The tubes l0 and 5 thus conduct alternately, the tube l0 conducting when the key of the keyer is down, and the tube 16 conducting when the key is up.

The tubes H! and [6 should be similar and matched so that they draw the same plate current when conducting. As a result, the D. C. voltage at their plates remains constant throughout the keying so that there are no pulses applied to the grid of the following tube I I. As a result, the output signal has the wave form shown by Fig. 7 of the drawing instead of that shown by Fig. 5.

In an actual application of this invention to a radio transmitter, it 'was possible to provide modulated, continuous wave keying at speeds of v up to one hundred words per minute.

It should be apparent that the tubes l and I6 could be tube unitscontained in one tube envelope. V

I claim as my invention:

1. An electronic keying circuit comprising two,

similar, electron tube units having platesv direc ly nected together, andhavin'g control grids; means for applying an input signal between the control grid and cathode of one of said units; an electron tube having'a. control grid, resistance coupled to said plates, and keying means for applying direct current biasing'pulses to the control grid of one of said units,'and for applying direct cur rent biasing pulses, out-of-phase with said pulses, to the control grid of'the other of said units.

3. An electronic keying circuit ma resistance coupled, speech amplifier of'a radio transmitter having an input, electron tube unit with a control grid and a cathode connected to receive an input signal, and having a plate, resistance coupled to the control grid of a following electron tube, c0mprising an electron tube unit similar to said input tube unit, and having its plate and cathode directly connected to the plate and cathode respectively, of said input tube unit, and keying means for applying direct current biasing pulses to the control grid of one of said units, and for applying direct current biasing pulses, out-of-phase with said pulses, to the control grid of the other of said units.

4. In combination with a resistance coupled speech amplifier of a radio transmitter, said amplifler'having an input electron tube unit with a control grid and a cathode connected to receive an input signal, and having a plate, resistance pled t0. thev control grid of a following electron tube; an electron tube unit similar to said input tube unit, and having a plate and a cathode directly connected to the plate and cathode respectively, of saidinput tube unit, and having a control grid, and keying means for applying direct current biasing pulses to the control grid ofone 'of said units, and for applying direct current biasing pulses, out-of-phase with said pulses, to the control grid of the other of said units.

' MARK I. JACOB.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,948,103 Finch Feb. 20, 1934 1,977,596 Pray Oct. 16, 1934 2,092,496 Branson Sept. 7, 1937 2,399,586 Toomim Apr. 30, 1946 2,400,822 Hansell et al May 21, 1946 2,405,876 Crosby Aug. 13, 1946 2,455,616 Shepard Dec. '7, i948 

